Electronic Device and Manufacturing Method Thereof

ABSTRACT

An object of the invention is to provide an electronic device which can be easily manufactured using a wet method. One of electronic devices according to the invention has a first layer and a second layer. The first layer contains a first compound including a conjugated double bond. Here, the first compound preferably has a molecular weight of 100 to 1000. The second layer contains a seconds compound having a cyclic structure which is formed by an addition reaction between two molecules of the first compound. Here, a light emitting element or an element such as a transistor can be given as the electronic device.

TECHNICAL FIELD

The present invention relates to an electronic device such as a lightemitting element or a semiconductor element, and particularly to astructure of a layer included in the electronic device, through which anelectric current passes when voltage is applied, and a manufacturingmethod thereof.

BACKGROUND ART

An inkjet apparatus which has been used as a printing apparatus is alsoused as an apparatus for forming a wiring or a film these days. Withsuch expansion of applications, the development of materials havingperformance appropriate for each application has been required.

In the development field of, for example, an electroluminescent elementor an organic transistor, a light emitting layer, a transport layer, asemiconductor layer, or the like is formed using a wet method such as aninkjet method or a coating method. As a material for forming theselayers, a high molecular weight compound is mainly used.

However, when film formation using a low molecular weight compound aswell as a high molecular weight compound by a wet method such as aninkjet method becomes easy, more various elements can be manufactured.

Therefore, a technique for manufacturing an element using a lowmolecular weight compound as well as a high molecular weight compound isunder development. For example, Reference 1 discloses a method forforming a thin film with an inkjet method using a low molecular weightcompound by providing a step of forming a self-assembled film (Reference1: Japanese Patent Laid-Open No. 2003-234522).

In addition, Reference 2 discloses that, when a light emitting layer isformed by a coating method using an organic solvent solution in which ahigh molecular weight light emitting material is dissolved, there is aproblem of invasion of a hole injection layer by the organic solvent(Reference 2: Japanese Patent Laid-Open No. 2003-163086). Further, italso discloses a light emitting element provided with a hole injectionlayer containing an organic solvent insoluble high molecular weightmolecule as its main component in order to solve the problem. Accordingto Reference 2, such a hole injection layer is formed by a method inwhich a film formed by applying a solution containing an initiator isirradiated with a mercury lamp. However, according to such a method,there is the case where the initiator remains in the film as animpurity.

DISCLOSURE OF INVENTION

It is an object of the invention to provide an electronic device whichcan be easily manufactured using a wet method.

One of electronic devices according to the invention is an electronicdevice having a layer containing a compound produced by a[2+2]cycloaddition reaction. Here, a light emitting element or asemiconductor element such as a transistor can be given as theelectronic device.

Another electronic device according to the invention has a first layerand a second layer. The first layer contains a first compound includinga conjugated double bond. Here, the first compound preferably has amolecular weight of 100 to 1000. The second layer contains a secondcompound having a cyclic structure which is formed by an additionreaction between two molecules of the first compound. Here, a lightemitting element or a semiconductor element such as a transistor can begiven as the electronic device.

One of methods for manufacturing an electronic device according to theinvention includes a first step and a second step. The first step is astep of forming a first layer containing a compound including aconjugated double bond. Here, the compound preferably has a molecularweight of 100 to 1000. The second step is a step of irradiating thefirst layer with light so as to cause a [2+2]cycloaddition reaction ofthe compound contained in the first layer. Here, a light emittingelement or a semiconductor element such as a transistor can be given asthe electronic device.

Another light emitting element according to the invention is a lightemitting element including a layer containing a compound produced by a[2+2]cycloaddition reaction between a first electrode and a secondelectrode.

Another light emitting element according to the invention is a lightemitting element including a first layer and a second layer between afirst electrode and a second electrode. The first layer contains a firstcompound. The second layer contains a second compound. Here, the firstcompound is a compound including a conjugated double bond. The firstcompound preferably has a molecular weight of 100 to 1000. The secondcompound is a compound having a cyclic structure which is formed by anaddition reaction between two molecules of the first compound.

Another method for manufacturing a light emitting element according tothe invention is a manufacturing method including the step of forming alayer containing a compound having a conjugated double bond and the stepof irradiating the layer with light to cause a [2+2]cycloadditionreaction. The compound having a conjugated double bond preferably has amolecular weight of 100 to 1000.

One of semiconductor elements according to the invention is a transistorincluding a layer containing a compound produced by a [2+2]cycloadditionreaction between a first semiconductor layer and a second semiconductorlayer.

Another semiconductor element according to the invention is a transistorincluding a first semiconductor layer, a second semiconductor layer, anda third semiconductor layer. The first semiconductor layer contains afirst compound. The second semiconductor layer contains a secondcompound. Here, the first compound is a compound including a conjugateddouble bond. The first compound preferably has a molecular weight of 100to 1000. The second compound is a compound having a cyclic structurewhich is formed by an addition reaction between two molecules of thefirst compound. Further, the polarity of a preferentially transportedcarrier differs between the first semiconductor layer and the thirdsemiconductor layer.

One of methods for manufacturing a semiconductor element according tothe invention is a manufacturing method including the step of forming alayer containing a compound having a conjugated double bond and the stepof irradiating the layer with light to cause a [2+2]cycloadditionreaction. The compound having a conjugated double bond preferably has amolecular weight of 100 to 1000.

According to the invention, an electronic device such as a lightemitting element or a semiconductor element, having a laminatedstructure of a plurality of layers formed using a wet method can beeasily manufactured. Further, an electronic device can be easilymanufactured at low cost by particularly using a drawing method among awet method. Moreover, according to the invention, an electronic devicewith low impurity content can be easily manufactured.

In addition, a low-cost and inexpensive light emitting device orsemiconductor device, or an electronic device which becomes low-cost andinexpensive by mounting the same can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a structure of a light emitting device of the presentinvention.

FIGS. 2A to 2D show a method for manufacturing a light emitting deviceof the present invention.

FIGS. 3A to 3C show a method for manufacturing a light emitting deviceof the present invention.

FIG. 4 shows a light emitting device to which the present invention isapplied.

FIG. 5 shows a circuit included in a light emitting device to which thepresent invention is applied.

FIG. 6 is a top view of a light emitting device to which the presentinvention is applied.

FIG. 7 shows frame operation of a light emitting device to which thepresent invention is applied.

FIGS. 8A to 8C are cross-sectional views of a light emitting device towhich the present invention is applied.

FIGS. 9A to 9C show electronic devices to which the present invention isapplied.

FIG. 10 shows a semiconductor device of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiment modes of the present invention will beexplained. However, the invention can be carried out in many differentmodes. As is easily known to a person skilled in the art, the mode andthe detail of the invention can be variously changed without departingfrom the spirit and the scope of the present invention. Thus, thepresent invention is not interpreted while limiting to the followingdescription of the embodiment mode.

Embodiment Mode 1

In this embodiment mode, a method for manufacturing a light emittingelement of the invention, having a plurality of layers between a firstelectrode 101 and a second electrode 102 as shown in FIG. 1, isexplained with reference to FIGS. 2A to 2D and 3A to 3C. Although fivelayers of a first layer 111, a second layer 112, a third layer 113, afourth layer 114, and a fifth layer 115 are laminated in FIG. 1, thenumber of laminated layers is not particularly limited.

A barrier layer 121 provided with an opening so as to expose a part ofthe first electrode 101 is formed over the first electrode 101.

Here, the first electrode 101 is not particularly limited and may beformed from a conductive material such as aluminum (Al), gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), or palladium (Pd) as well asindium tin oxide, indium tin oxide containing silicon oxide, or indiumoxide containing zinc oxide of 2% to 20%. The barrier layer 121 is alsonot particularly limited and may be formed from an inorganic materialsuch as silicon oxide, an organic material such as acrylic, polyimide,or a resist, or the like. The barrier layer 121 may alternatively beformed using siloxane or the like.

Subsequently, a first layer 111 containing a compound including aconjugated double bond (first compound) is formed over the firstelectrode 101. The first compound is preferably a low molecular weightcompound which can be easily produced by a [2+2]cycloaddition reaction.Here, the low molecular weight compound means a compound having amolecular weight of 100 to 1000. As such a compound, for example,anthracene, an anthracene derivative, cinnamic acid, a cinnamic acidderivative, a coumarin derivative, or the like can be given.

A method for forming the first layer 111 is not particularly limited.The first layer 111 may be formed by any of methods such as anevaporation method, a coating method, and a drawing method. Here, thedrawing method means a method for selectively forming a film in adesired portion while controlling the timing or position of dropping amaterial solution. Note that, by using the drawing method, a substancethat is a material for forming the layer can be used without waste, anda light emitting element can be manufactured with high material-useefficiency.

Subsequently, the first layer 111 is irradiated with light to cause a[2+2]cycloaddition reaction of the first compound. Here, the[2+2]cycloaddition reaction is one kind of photoreactions and means thereaction of compounds each including a conjugated double bond to form acyclic structure by addition.

By causing a [2+2]cycloaddition reaction of the first compound, a secondcompound that is a photodimer of the first compound is produced. Whenthe first compound is, for example, anthracene, a photodimer ofanthracene is produced as the second compound by light irradiation. Inthis manner, a second layer 112 containing the second compound isformed.

A light irradiation method or the like is not particularly limited. Thewavelength, irradiation time, irradiation intensity, or the like ofirradiation light may be adjusted in accordance with characteristics ofthe first compound so as to cause a [2+2]cycloaddition reaction.Further, there is no particular limitation on how deep from the surface(surface on a light entering side) in a thickness direction a region inthe first layer 111 is changed to the second layer 112.

The second layer 112 formed as described above is a layer having lowersolubility in a solvent, particularly, an organic solvent than the firstlayer 111.

Subsequently, a third layer 113 containing a compound including aconjugated double bond (third compound) is formed over the second layer112. The third compound is preferably a low molecular weight compoundwhich can be easily produced by a [2+2]cycloaddition reaction, and asimilar compound to the first compound can be used.

A method for manufacturing the third layer 113 is not particularlylimited. The third layer 113 may be formed by any of methods such as anevaporation method, a coating method, and a drawing method. Since thesecond layer 112 is hard to dissolve in a solvent, particularly, anorganic solvent, a layer can be easily formed over the second layer 112not only by a dry method such as an evaporation method but also by a wetmethod such as a coating method or a drawing method using a solutioncontaining a solvent, particularly, an organic solvent as a material. Byusing the drawing method, a substance that is a material for forming thelayer can be used without waste, and a light emitting element can bemanufactured with high material-use efficiency.

Then, the third layer 113 is irradiated with light to cause a[2+2]cycloaddition reaction of the third compound. By causing a[2+2]cycloaddition reaction of the third compound, a fourth compoundthat is a dimer of the third compound is produced. In this manner, afourth layer 114 containing the fourth compound is formed.

A light irradiation method or the like is not particularly limited. Thewavelength, irradiation time, irradiation intensity, or the like ofirradiation light may be adjusted in accordance with characteristics ofthe third compound to cause a [2+2]cycloaddition reaction. Further,there is no particular limitation on how deep from the surface (surfaceon a light entering side) in a thickness direction a region in the thirdlayer 113 is changed to the fourth layer 114.

Next, a fifth layer 115 is formed over the fourth layer 114. A methodfor forming the fifth layer 115 is not particularly limited. The fifthlayer 115 may be formed by any of methods such as an evaporation method,a coating method, and a drawing method. Since the fourth layer 114 is alayer formed of the fourth compound produced by a [2+2]cycloadditionreaction, a layer can be easily formed over the fourth layer 114 notonly by a dry method such as an evaporation method but also by a wetmethod such as a coating method or a drawing method. By using thedrawing method, a substance that is a material for forming the layer canbe used without waste, and a light emitting element can be manufacturedwith high material-use efficiency. The fifth layer 115 may be formedusing a high molecular weight compound as well as a low molecular weightcompound. Here, the high molecular weight compound is a compound havinga synchronization structure within a molecule and having a distributionin molecular weight. When a layer containing a substance produced by a[2+2]cycloaddition reaction need not particularly be provided, the fifthlayer 115 may be formed using a high molecular weight compound as wellas a low molecular weight compound as described above. However, when alow molecular weight compound is used, a light emitting element, whichdoes not contain an impurity such as a polymerization initiator, can beobtained.

By providing the second layer 112 or the fourth layer 114 containing acompound produced by a [2+2]cycloaddition reaction as described above,it becomes easy to form the third layer 113 or the fifth layer 115 by awet method. Thus, the light emitting element of the invention can beeasily manufactured by a wet method; therefore, it can be manufacturedat low cost by forming a film particularly using a drawing method. Notethat an inkjet method or the like can be given as a specific example ofthe drawing method.

Subsequently, a second electrode 102 is formed over the fifth layer 115.Here, the second electrode 102 is not particularly limited and may beformed from a conductive material such as aluminum (Al), gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), or palladium (Pd) as well asindium tin oxide, indium tin oxide containing silicon oxide, indiumoxide containing zinc oxide of 2% to 20%. Note that either or both thefirst electrode 101 and the second electrode 102 are preferably formedfrom a conductive material which can transmit visible light. This makesit possible to extract emitted light through either or both electrodes.

In this embodiment mode, treatment for causing a [2+2]cycloadditionreaction is performed on the two layers, the first layer 111 and thethird layer 113. However, such treatment need not be performed on alllayers formed by a film forming method such as an evaporation method, acoating method, or a drawing method. For example, when the thirdcompound is insoluble in the solvent included in the fifth layer 115,treatment for causing a [2+2]cycloaddition reaction may be performedonly on the first layer 111 because the fourth layer 114 need notnecessarily be formed.

When voltage is applied to the first electrode 101 and the secondelectrode 102 and a current flows therebetween, an electron and a holeare recombined to excite a light emitting substance. The excited lightemitting substance emits light when returning to a ground state. Thereis no particular limitation on in which layer a light emitting substanceis included. In such a light emitting element as shown in FIG. 1, alight emitting substance is preferably included in the third layer 113which is away from both electrodes. This can prevent quenching due tometal. Here, the light emitting substance is a substance that hasfavorable luminous efficiency and can emit light of a desired emissionwavelength. Therefore, when the third compound can react by[2+2]cycloaddition and has favorable luminous efficiency, the thirdcompound may be used as the light emitting substance. When a differentmaterial from the third compound is made to emit light, a substancewhich can emit light of a desired emission wavelength may be mixedtogether with the third compound. There is no particular limitation onthe light emitting substance, and a phosphorescent substance or the likeas well as a fluorescent substance can be used. The thickness, carriertransport properties, or the like of the first layer 111, the secondlayer 112, the fourth layer 114, and the fifth layer 115 may be adjustedso as to recombine an electron and a hole in the third layer 113. Inaddition, a layer containing alkali metal or alkaline earth metal suchas lithium fluoride, calcium fluoride, lithium, or calcium, may beprovided to be in contact with one of the two electrodes, which servesas a cathode, in order to assist electron injection. In addition, alayer containing metal oxide such as molybdenum oxide or vanadium oxide,or the like may be provided to be in contact with the other electrodewhich serves as an anode in order to assist hole injection.

A light emitting element, in which a plurality of layers formed by a wetmethod is laminated, can be easily manufactured by carrying out theabove-described method for manufacturing the light emitting element ofthe invention. It can also be manufactured at low cost when manufacturedparticularly using a drawing method among a wet method. Further, byemploying the method for manufacturing a light emitting element of theinvention, a light emitting element, which does not contain an impuritysuch as a polymerization initiator, can be manufactured.

Embodiment Mode 2

In this embodiment mode, a semiconductor element to which the inventionis applied is explained with reference to FIG. 10.

In FIG. 10, a gate insulating layer 202 is provided to cover a gateelectrode 201 over a support 200. The gate electrode 201 is notparticularly limited, and can be formed using a conductive material suchas aluminum, copper, gold, or silver. The gate insulating layer 202 isalso not particularly limited, and may be formed of an organic materialas well as an inorganic material such as silicon oxide or siliconnitride. Further, the support 200 is not particularly limited, and aflexible substrate such as a plastic substrate as well as a glasssubstrate, a quartz substrate, or the like may be used.

A first semiconductor layer 203 is provided over the gate insulatinglayer 202 to overlap the gate electrode 201 and the gate insulatinglayer 202. The first semiconductor layer 203 is a layer containing acompound including a conjugated double bond (fifth compound). The fifthcompound is preferably a low molecular weight compound which can beeasily produced by a [2+2]cycloaddition reaction. Here, the lowmolecular weight compound means a compound having a molecular weight of100 to 1000. For example, anthracene, an anthracene derivative, cinnamicacid, a cinnamic acid derivative, a coumarin derivative, a pentacenederivative, or the like can be given as such a compound.

A method for forming the first semiconductor layer 203 is notparticularly limited. The first semiconductor layer 203 may be formed byany of methods such as an evaporation method, a coating method, and adrawing method. Note that, by using the drawing method, a substance thatis a material for forming the layer can be used without waste, and atransistor can be manufactured with high material-use efficiency.

The transistor in FIG. 10 has a second semiconductor layer 204 incontact with the first semiconductor layer 203. The second semiconductorlayer 204 is a layer containing a dimer of the fifth compound (sixthcompound) produced by irradiating the first semiconductor layer 203 withlight and causing a [2+2]cycloaddition reaction. The secondsemiconductor layer 204 formed in this manner has the property of beinghard to dissolve in a solvent, particularly, an organic solvent.

A third semiconductor layer 205 is further provided over the secondsemiconductor layer 204. A method for forming the third semiconductorlayer 205 is not particularly limited. The third semiconductor layer 205may be formed by any of methods such as an evaporation method, a coatingmethod, and a drawing method. Since the second semiconductor layer 204is hard to dissolve in a solvent, particularly, an organic solvent, alayer can be easily formed over the second semiconductor layer 204 notonly by a dry method such as an evaporation method but also by a wetmethod such as a coating method or a drawing method using a solutioncontaining a solvent, particularly, an organic solvent as a material. Byusing the drawing method, a substance that is a material for forming thelayer can be used without waste, and a transistor can be manufacturedwith high material-use efficiency.

The first semiconductor layer 203 or the third semiconductor layer 205can be formed using a low molecular weight compound or a high molecularweight compound such as pentacene or polythiophene.

Here, one of the first semiconductor layer 203 and the thirdsemiconductor layer 205 is formed with an n-type semiconductor (asemiconductor in which the mobility of an electron is higher than thatof a hole and an electron is preferentially transported), and the otheris formed with a p-type semiconductor (a semiconductor in which themobility of a hole is higher than that of an electron and a hole ispreferentially transported). In other words, the polarity of apreferentially transported carrier differs between the firstsemiconductor layer 203 and the third semiconductor layer 205.

A source electrode 206 and a drain electrode 207 are provided over thethird semiconductor layer 205. The source electrode 206 and the drainelectrode 207 are not particularly limited, and may be formed using aconductive organic material formed with apoly(ethylenedioxythiophene)/poly(styrenesulfonate) water solution(PEDOT/PSS) or the like as well as aluminum, copper, gold, silver, orthe like. A method for forming the source electrode 206 and the drainelectrode 207 is not particularly limited. The source electrode 206 andthe drain electrode 207 may be formed by any of methods such as anevaporation method, a coating method, and a drawing method. In the caseof using a wet method such as an application method or a drawing method,a layer which is hard to dissolve in a solvent, particularly, an organicsolvent may be formed by irradiating the third semiconductor layer 205and causing a [2+2]cycloaddition reaction.

In the above-described transistor of the invention, voltage is appliedso as to make a potential difference between the source electrode 206and the drain electrode 207, and positive voltage is applied to the gateelectrode 201. Then, a channel is formed in the layer containing ann-type semiconductor and current flows therethrough. Voltage is appliedso as to make a potential difference between the source electrode 206and the drain electrode 207, and negative voltage is applied to the gateelectrode 201. Then, a channel is formed in the layer including a p-typesemiconductor and current flows therethrough. Specifically, in the casewhere the first semiconductor layer is formed with an n-typesemiconductor and the third semiconductor layer is formed with a p-typesemiconductor, a channel is formed in the first semiconductor layer 203when positive voltage is applied to the gate electrode 201. In addition,a channel is formed in the third semiconductor layer 205 when negativevoltage is applied to the gate electrode 201.

A light emitting element, in which a plurality of layers formed by a wetmethod is laminated, can be easily manufactured by carrying out theabove-described method for manufacturing the semiconductor element ofthe invention. It can also be manufactured at low cost particularly whenmanufactured using a drawing method among a wet method. Note that thestructure of the semiconductor element is not limited to that shown inFIG. 10, and a structure different from FIG. 10 may also be used.

Embodiment Mode 3

Since the light emitting element of the invention can be manufactured atlow cost, an inexpensive light emitting device, semiconductor device, orthe like can be manufactured by using the light emitting element of theinvention as a pixel or the like. In addition, a light emitting devicewith few defects in the light emitting element due to an impurity suchas a polymerization initiator can be obtained by carrying out theinvention.

In this embodiment mode, a circuit structure and a driving method of alight emitting device including the light emitting element of theinvention and having a display function are described with reference toFIGS. 4 to 7.

FIG. 4 is a schematic top view of a light emitting device to which theinvention is applied. In FIG. 4, a pixel portion 6511, a source signalline driver circuit 6512, a write gate signal line driver circuit 6513,and an erase gate signal line driver circuit 6514 are provided over asubstrate 6500. Each of the source signal line driver circuit 6512, thewrite gate signal line driver circuit 6513, and the erase gate signalline driver circuit 6514 is connected to a flexible printed circuit(FPC) 6503, which is an external input terminal, through a wiring group.Each of the source signal line driver circuit 6512, the write gatesignal line driver circuit 6513, and the erase gate signal line drivercircuit 6514 receives a video signal, a clock signal, a start signal, areset signal, or the like from the FPC 6503. The FPC 6503 is providedwith a printed wiring board (PWB) 6504. Note that a driver circuitportion need not necessarily be provided over the same substrate as thepixel portion 6511 as described above, and may be provided outside ofthe substrate by using, for example, an IC chip mounted on an FPC whichis provided with a wiring pattern (TCP) or the like.

In the pixel portion 6511, a plurality of source signal lines extendingin a column direction is arranged in a row direction. A current supplyline is also arranged in a row direction. In addition, a plurality ofgate signal lines extending in a row direction is arranged in a columndirection in the pixel portion 6511. Further, plural sets of circuitseach including a light emitting element are arranged in the pixelportion 6511.

FIG. 5 shows a circuit for operating one pixel. The circuit shown inFIG. 5 includes a first transistor 901, a second transistor 902, and alight emitting element 903.

Each of the first transistor 901 and the second transistor 902 is athree-terminal element including a gate electrode, a drain region, and asource region, and includes a channel region between the drain regionand the source region. The source region and the drain region areswitched depending on a structure or an operating condition of thetransistor, or the like; thus, it is difficult to determine which is thesource region or the drain region. Therefore, in this embodiment mode,regions each serving as a source or a drain are referred to as a firstelectrode and a second electrode.

A gate signal line 911 and a write gate signal line driver circuit 913are provided to be electrically connected or not to be electricallyconnected with each other via a switch 918. The gate signal line 911 andan erase gate signal line driver circuit 914 are provided to beelectrically connected or not to be electrically connected with eachother via a switch 919. A source signal line 912 is provided to beelectrically connected to either a source signal line driver circuit 915or a power source 916 via a switch 920. The gate of the first transistor901 is electrically connected to the gate signal line 911. The firstelectrode of the first transistor 901 is electrically connected to thesource signal line 912, and the second electrode of the first transistor901 is electrically connected to the gate electrode of the secondtransistor 902. The first electrode of the second transistor 902 iselectrically connected to a current supply line 917, and the secondelectrode of the second transistor 902 is electrically connected to oneelectrode included in the light-emitting element 903. The switch 918 maybe included in the write gate signal line driver circuit 913. Further,the switch 919 may be included in the erase gate signal line drivercircuit 914. Moreover, the switch 920 may be included in the sourcesignal line driver circuit 915.

The arrangement of the transistor, the light emitting element, and thelike in the pixel portion is not particularly limited. For example, theelements can be arranged as shown in a top view of FIG. 6. In FIG. 6, afirst electrode of a first transistor 1001 is connected to a sourcesignal line 1004, and a second electrode of the first transistor 1001 isconnected to a gate electrode of a second transistor 1002. A firstelectrode of the second transistor 1002 is connected to a current supplyline 1005, and a second electrode of the second transistor is connectedto an electrode 1006 of a light emitting element. A part of a gatesignal line 1003 serves as a gate electrode of the first transistor1001.

Next, a driving method is explained. FIG. 7 is an explanatory view of anoperation of a frame with time. In FIG. 7, the abscissa-axis directionrepresents time passage, whereas the ordinate-axis direction representsscanning stages of a gate signal line.

When an image is displayed with a light emitting device of theinvention, a rewriting operation and a displaying operation arerepeatedly carried out in a display period. The number of rewritingoperations is not particularly limited; however, the rewriting operationis preferably performed approximately sixty times per second so that aperson who watches the image does not realize flickering. Herein, theperiod for performing the rewriting and displaying operations of oneimage (one frame) is referred to as a one frame period.

The one frame period is divided into four subframe periods 501, 502,503, and 504 including writing periods 501 a, 502 a, 503 a, and 504 a,and retention periods 501 b, 502 b, 503 b, and 504 b. A light-emittingelement provided with a light-emission signal emits light in theretention period. The length ratio of the retention periods in the firstsubframe period 501, the second subframe period 502, the third subframeperiod 503, and the fourth subframe period 504 is 2³:2²:2¹:2⁰=8:4:2:1.Accordingly, a 4-bit gray scale can be realized. The number of bits orgray scale levels is not limited thereto. For instance, an 8-bit grayscale can be achieved by providing eight subframe periods.

An operation in the one frame period is explained. First, a writingoperation is carried out sequentially from the first row to the last rowin the subframe period 501. Therefore, the starting time of a writingperiod differs depending on the rows. The retention period 501 b startsin the row where the writing period 501 a is completed. In the retentionperiod, a light emitting element provided with a light emission signalemits light. The subframe period 502 starts in the row where theretention period 501 b is completed, and a writing operation is carriedout sequentially from the first row to the last row as is the case withthe subframe period 501. Operations as noted above are repeatedlycarried out until the retention period 504 b of the subframe period 504is completed. When the operation in the subframe period 504 iscompleted, an operation in the next frame period is started. The sum oflight emitting time in each subframe period is light emitting time ofeach light emitting element in the one frame period. By changing theemitting time for each light emitting element and variously combining inone pixel, various colors can be displayed with different brightness andchromaticity.

When a retention period in the row where writing has been finished andthe retention period has started is intended to be forcibly terminatedbefore finishing the writing of the last row, like in the subframeperiod 504, preferably an erasing period 504 c is provided after theretention period 504 b to control the operations so that the lightemission is forcibly stopped. In the row where the light-emission isforcibly stopped, the light emitting element does not emit light for acertain period (the period is referred to as a non-light emission period504 d). Upon finishing the writing period of the last row, the nextwriting period (or frame period) starts sequentially from the first row.This can prevent the writing period of the subframe period 504 fromoverlapping the writing period of the next subframe period.

In this embodiment mode, the subframe periods 501 to 504 are arranged tocoincide in the order of decreasing retention periods; however, thepresent invention is not limited thereto. For instance, the subframeperiods 501 to 504 may be arranged to coincide in the order ofincreasing retention periods. The subframe periods 501 to 504 may bearranged at random, mixing a short subframe period and a long subframeperiod. The subframe period may be further divided into a plurality offrame periods. In other words, scanning of the gate signal line can becarried out a plurality of times during the period of giving the samevideo signal.

An operation of the circuit shown in FIG. 5 in a writing period and anerasing period is explained.

First, an operation in the writing period is explained. In the writingperiod, the gate signal line 911 in the n-th row (n is a natural number)is electrically connected to the write gate signal line driver circuit913 via the switch 918, and is not connected to the erase gate signalline driver circuit 914. The source signal line 912 is electricallyconnected to the source signal line driver circuit 915 via the switch920. A signal is inputted to the gate of the first transistor 901connected to the gate signal line 911 in the n-th row, and the firsttransistor 901 is turned ON. At this time, video signals aresimultaneously inputted to the source signal lines in the first columnto the last column. Video signals inputted from the source signal line912 at each column are independent of each other. The video signalinputted from the source signal line 912 is inputted to the gateelectrode of the second transistor 902 via the first transistor 901connected to each source signal line. At this time, a current value tobe supplied from the current supply line 917 to the light emittingelement 903 is determined depending on a signal inputted to the secondtransistor 902. Then, emission or non-emission of the light-emittingelement 903 is determined depending on the current value. For example,in the case where the second transistor 902 is a p-channel type, thelight emitting element 903 emits light when a Low Level signal isinputted to the gate electrode of the second transistor 902. On theother hand, in the case where the second transistor 902 is an n-channeltype, the light emitting element 903 emits light when a High Levelsignal is inputted to the gate electrode of the second transistor 902.

Then, an operation in the erasing period is explained. In the erasingperiod, the gate signal line 911 in the n-th row (n is a natural number)is electrically connected to the erase gate signal line driver circuit914 via the switch 919, and is not connected to the write gate signalline driver circuit 913. The source signal line 912 is electricallyconnected to the power source 916 via the switch 920. A signal isinputted to the gate of the first transistor 901 connected to the gatesignal line 911 in the n-th row, and the first transistor 901 is turnedON. At this time, erase signals are simultaneously inputted to thesource signal lines in the first column to the last column. The erasesignal inputted from the source signal line 912 is inputted to the gateelectrode of the second transistor 902 via the first transistor 901connected to each source signal line. By the signal inputted to thesecond transistor 902, current supply from the current supply line 917to the light emitting element 903 is stopped. Then, the light emittingelement 903 does not emit light forcibly. For example, in the case wherethe second transistor 902 is a p-channel type, the light emittingelement 903 does not emit light when a High Level signal is inputted tothe gate electrode of the second transistor 902. On the other hand, inthe case where the second transistor 902 is an n-channel type, the lightemitting element 903 does not emit light when a Low Level signal isinputted to the gate electrode of the second transistor 902.

In the erasing period, a signal for erasing is inputted to the n-th row(n is a natural number) by the operation as described above. However, asdescribed above, there is a case that the n-th row is in an erasingperiod and another row (the m-th row (m is a natural number) in thisinstance) is in a writing period. In this case, it is required that asignal for erasing is inputted to the n-th row and a signal for writingis inputted to the m-th row by utilizing a source signal line of thesame column. Accordingly, an operation to be explained as follows ispreferably carried out.

Immediately after the light emitting element 903 in the n-th row isbrought into a non emission state by the operation in the erasing perioddescribed above, the gate signal line is disconnected from the erasegate signal line driver circuit 914, and the source signal line isconnected to the source signal line driver circuit 915 by operating theswitch 920. In addition to connecting the source signal line to thesource signal line driver circuit 915, the gate signal line is connectedto the write gate signal line driver circuit 913. A signal isselectively inputted to the signal line in the m-th row from the writegate signal line driver circuit 913, and the first transistor is turnedON. In addition, signals for writing are inputted to the source signallines in the first column to the last column from the source signal linedriver circuit 915. The light emitting element in the m-th row emitslight or no light depending on the signal.

Upon finishing the writing period of the m-th row as noted above, anerasing period in the (n+1)-th row starts. Hence, the gate signal lineis disconnected from the write gate signal line driver circuit 913, andthe source signal line is connected to the power source 916 by operatingthe switch 920. Further, the gate signal line is disconnected from thewrite gate signal line driver circuit 913, and the gate signal line isconnected to the erase gate signal line driver circuit 914. When asignal is selectively inputted to the gate signal line in the (n+1)-throw from the erase gate signal line driver circuit 914 and the firsttransistor is turned ON, an erase signal is inputted from the powersource 916. Upon finishing the erasing period of the (n+1)-th row, awriting period in the m-th row starts. Hereinafter, in the similarmanner, an erasing period and a writing period may be carried outrepeatedly to operate to an erasing period of the last row.

In this embodiment mode, a mode in which the writing period of the m-throw is provided between the erasing period of the n-th row and theerasing period of the (n+1)-th row is explained. Without being limitedthereto, however, the writing period of the m-th row may be providedbetween the erasing period of the (n−1)-th row and the erasing period ofthe n-th row.

In this embodiment mode, when the non-light emission period 504 d isprovided as in the subframe period 504, an operation of disconnectingthe erase gate signal line driver circuit 914 from a certain gate signalline and connecting the write gate signal line driver circuit 913 toanother gate signal line is repeatedly carried out. Such an operationmay be carried out in a frame period that is not provided with anon-light emission period.

Embodiment Mode 4

An example of a cross-sectional view of a light emitting deviceincluding an electronic device of the invention is explained withreference to FIGS. 8A to 8C.

In FIGS. 8A to 8C, a portion surrounded by dotted lines is a transistor11 which is provided to drive a light emitting element 12 of theinvention. The light emitting element 12 is the light emitting elementof the invention having a layer 15 in which a plurality of layers islaminated between a first electrode 13 and a second electrode 14 asdescribed in Embodiment Mode 1. A drain of the transistor 11 iselectrically connected to the first electrode 13 by a wiring 17penetrating a first interlayer insulating film 16 (16 a, 16 b, and 16c). The light emitting element 12 is separated from another adjacentlyprovided light emitting element by a barrier layer 18. The lightemitting device of the invention having a structure such as this isprovided over a substrate 10 in this embodiment mode.

Note that each of the transistors 11 shown in FIGS. 8A to 8C is a topgate type in which a gate electrode is provided opposite to a substrate,with a semiconductor layer interposed therebetween. However, there is noparticular limitation on the structure of the transistor 11; forexample, a bottom gate type may be used. In the case of a bottom gatetype, the transistor 11 may have a structure in which a protective filmis formed over the semiconductor layer to be provided with a channel (achannel protective type) or a structure in which a part of thesemiconductor layer to be provided with a channel has a depression (achannel etch type).

Alternatively, the semiconductor layer included in the transistor 11 maybe either crystalline or amorphous. Further, it may be semi-amorphous.Still alternatively, it may be a semiconductor layer including asemiconductor formed of an organic material in addition to asemiconductor formed of an inorganic material.

Note that characteristics of the semi-amorphous semiconductor are asfollows. It has an intermediate structure between an amorphous structureand a crystalline structure (including a single crystal and apolycrystal) and a third state which is stable in terms of free energy,and it includes a crystalline region having short-range order andlattice distortion. At least a part of a region in the film contains acrystal grain of 0.5 nm to 20 nm. A Raman spectrum is shifted to a lowerwavenumber side than 520 cm⁻¹. A diffraction peak of (111) or (220) tobe caused by a crystal lattice of silicon is observed in X-raydiffraction. Hydrogen or halogen of 1 atomic % or more is included toterminate a dangling bond. It is also referred to as a microcrystalsemiconductor. The semi-amorphous semiconductor is formed by performingglow discharge decomposition (plasma CVD) on a silicide gas. SiH₄ isgiven as the silicide gas. In addition, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄,SiF₄, or the like can also be used as the silicide gas. The silicide gasmay be diluted with H₂, or H₂ and one or more rare gas elements ofhelium, argon, krypton, and neon. A dilution ratio thereof may rangefrom 2 times to 1000 times; pressures, approximately 0.1 Pa to 133 Pa;power supply frequency, 1 MHz to 120 MHz, preferably, 13 MHz to 60 MHz.A substrate heating temperature may be 300° C. or less, preferably, 100°C. to 250° C. An impurity concentration of an atmospheric constituentimpurity such as oxygen, nitrogen, or carbon, as an impurity element inthe film, is preferably 1×10²⁰/cm³ or less; specifically, aconcentration of oxygen is 5×10¹⁹/cm³ or less, preferably 1×10¹⁹/cm³ orless. Note that the mobility of a TFT (thin film transistor) using thesemi-amorphous semiconductor is approximately 1 cm²/Vsec to 10 cm²/Vsec.

As a specific example of the crystalline semiconductor layer, a layerformed of single-crystal or polycrystalline silicon, silicon germanium,or the like can be given. It may be formed by laser crystallization ormay be formed by crystallization through a solid phase growth methodusing, for example, nickel.

When the semiconductor layer is formed of an amorphous substance, forexample, amorphous silicon, a light emitting device preferably has acircuit in which the transistor 11 and all other transistors(transistors included in a circuit for driving a light emitting element)are all n-channel transistors. Other than that, a light emitting devicemay have a circuit including either n-channel transistors or p-channeltransistors, or a light emitting device may have a circuit includingboth types of transistors.

The first interlayer insulating film 16 may have a multi-layer structureas shown in FIGS. 8A to 8C, or a single-layer structure. Note that theinterlayer insulating film 16 a is made from an inorganic material suchas silicon oxide or silicon nitride; the interlayer insulating film 16 bis made from acrylic, siloxane (a compound having a skeleton formed froma bond of silicon (Si) and oxygen (O) and has a substituent of an alkylgroup or the like), or a self-planarizing substance which can form afilm by coating, such as silicon oxide. In addition, the interlayerinsulating film 16 c is made from a silicon nitride film containingargon (Ar). Note that there is no particular limitation on materialsforming each layer, and a material other than the foregoing materialscan also be used. A layer made from a material other than the foregoingmaterials may also be combined. As described above, the first interlayerinsulating film 16 may be formed with either an inorganic material or anorganic material, or both of them.

The barrier layer 18 preferably has a shape in an edge portion, in whicha curvature radius changes continuously. In addition, the barrier layer18 is formed with acrylic, siloxane, a resist, silicon oxide, or thelike. Note that the barrier layer 18 may be formed with either aninorganic material or an organic material, or both of them.

In FIGS. 8A and 8C, only the first interlayer insulating film 16 isprovided between the transistor 11 and the light emitting element 12.However, as shown in FIG. 8B, a second interlayer insulating film 19 (19a and 19 b) may also be provided in addition to the first interlayerinsulating film 16 (16 a and 16 b). In the light emitting device shownin FIG. 8B, the first electrode 13 penetrates the second interlayerinsulating film 19 and connects to the wiring 17.

The second interlayer insulating film 19 may have a multi-layerstructure or a single-layer structure like the first interlayerinsulating film 16. The second interlayer insulating film 19 a is madefrom acrylic, siloxane, or a self-planarizing substance which can form afilm by coating, such as silicon oxide. The second interlayer insulatingfilm 19 b is formed with a silicon nitride film containing argon (Ar).Note that there is no particular limitation on materials forming eachlayer, and a material other than the foregoing materials can also beused. A layer made from a material other than the foregoing materialsmay also be combined. As described above, the second interlayerinsulating film 19 may be formed with either an inorganic material or anorganic material, or both of them.

When both the first electrode 13 and the second electrode 14 are formedfrom a light transmitting material in the light emitting element 12,light emission can be extracted through both the first electrode 13 andthe second electrode 14 as indicated by the outlined arrow in FIG. 8A.When only the second electrode 14 is formed from a light transmittingmaterial, light emission can be extracted only through the secondelectrode 14 as indicated by the outlined arrow in FIG. 8B. In thiscase, it is preferable to form the first electrode 13 from a highlyreflective material or provide a film formed from a highly reflectivematerial (reflective film) below the first electrode 13. When only thefirst electrode 13 is formed from a light transmitting material, lightemission can be extracted only through the first electrode 13 asindicated by the outlined arrow in FIG. 8C. In this case, it ispreferable to form the second electrode 14 from a highly reflectivematerial or provide a reflective film above the second electrode 14.

In the light emitting element 12, the layer 15 may have such a laminatedstructure as to operate the light emitting element 12 when voltage isapplied so that a potential of the second electrode 14 becomes higherthan that of the first electrode 13, or the layer 15 may have such alaminated structure as to operate the light emitting element 12 whenvoltage is applied so that a potential of the second electrode 14becomes lower than that of the first electrode 13. In the former case,the transistor 11 is an n-channel transistor, and in the latter case,the transistor 11 is a p-channel transistor.

As described above, an active light emitting device in which drive ofthe light emitting element is controlled by the transistor is explainedin this embodiment mode. However, a passive light emitting device, inwhich the light emitting element is driven without particularlyproviding a drive element such as a transistor, may also be used. Thepassive light emitting device can also be driven with low powerconsumption when it includes the light emitting element of the inventionwhich operates at low drive voltage.

Embodiment Mode 5

An electronic device can be obtained at low cost by mounting the lightemitting device of the invention, since the light emitting device of theinvention can be manufactured at low cost.

FIGS. 9A to 9C show embodiments of an electronic device mounted with thelight emitting device to which the invention is applied.

FIG. 9A shows a notebook personal computer manufactured by applying theinvention, which includes a main body 5521, a chassis 5522, a displayportion 5523, a keyboard 5524, or the like. The personal computer can becompleted by incorporating a light emitting device having the lightemitting element of the invention as a display portion.

FIG. 9B shows a telephone manufactured by applying the invention, whichincludes a main body 5552, a display portion 5551, an audio outputportion 5554, an audio input portion 5555, operation keys 5556 and 5557,an antenna 5553, or the like. The telephone can be completed byincorporating a light emitting device having the light emitting elementof the invention as a display portion.

FIG. 9C shows a television set manufactured by applying the invention,which includes a display portion 5531, a chassis 5532, a loudspeaker5533, or the like. The television set can be completed by incorporatinga light emitting device having the light emitting element of theinvention as a display portion.

As describe above, the light emitting device of the invention is verysuitable for use as a display portion of various electronic devices.

Note that the light emitting device having the light emitting element ofthe invention may also be mounted on a navigation system, a lightingdevice, or the like, other than the above-mentioned electronic devices.

1. A light emitting device comprising: a first layer which contains afirst compound including a conjugated double bond and having a molecularweight of 100 to 1000; and a second layer containing a second compoundhaving a cyclic structure which is formed by an addition reaction of thefirst compound, wherein the first layer and the second layer arelaminated.
 2. The light emitting device according to claim 1, whereinsaid first compound is selected from the group consisting of anthracene,an anthracene derivative, cinnamic acid, a cinnamic acid derivative, acoumarin derivative and a pentacene derivative.
 3. An electronic devicehaving at least the light emitting device according to claim 1, whereinsaid electronic device is selected from the group consisting of anotebook personal computer, a telephone, a television set, a navigationsystem and a lighting device.
 4. A light emitting device comprising afirst layer and a second layer between a first electrode and a secondelectrode, wherein the first layer contains a first compound including aconjugated double bond and having a molecular weight of 100 to 1000, andthe second layer contains a second compound having a cyclic structurewhich is formed by an addition reaction of the first compound.
 5. Thelight emitting device according to claim 4, wherein said first compoundis selected from the group consisting of anthracene, an anthracenederivative, cinnamic acid, a cinnamic acid derivative, a coumarinderivative and a pentacene derivative.
 6. An electronic device having atleast the light emitting device according to claim 4, wherein saidelectronic device is selected from the group consisting of a notebookpersonal computer, a telephone, a television set, a navigation systemand a lighting device.
 7. A light emitting device comprising a secondsemiconductor layer between a first semiconductor layer and a thirdsemiconductor, wherein the first semiconductor layer contains a firstcompound including a conjugated double bond and having a molecularweight of 100 to 1000, the second semiconductor layer contains a secondcompound having a cyclic structure which is formed by an additionreaction of the first compound, and polarity of a preferentiallytransported carrier differs between the third semiconductor layer andthe first semiconductor layer.
 8. The light emitting device according toclaim 7, wherein said first compound is selected from the groupconsisting of anthracene, an anthracene derivative, cinnamic acid, acinnamic acid derivative, a coumarin derivative and a pentacenederivative.
 9. An electronic device having at least the light emittingdevice according to claim 7, wherein said electronic device is selectedfrom the group consisting of a notebook personal computer, a telephone,a television set, a navigation system and a lighting device.
 10. A lightemitting device comprising a plurality of arranged light emittingelements, wherein the light emitting element includes: a first layerwhich contains a first compound including a conjugated double bond andhaving a molecular weight of 100 to 1000; and a second layer containinga second compound having a cyclic structure which is formed by anaddition reaction of the first compound, in which the first layer andthe second layer are laminated.
 11. The light emitting device accordingto claim 10, wherein said first compound is selected from the groupconsisting of anthracene, an anthracene derivative, cinnamic acid, acinnamic acid derivative, a coumarin derivative and a pentacenederivative.
 12. An electronic device having at least the light emittingdevice according to claim 10, wherein said electronic device is selectedfrom the group consisting of a notebook personal computer, a telephone,a television set, a navigation system and a lighting device.
 13. Anelectronic device comprising the light emitting device according toclaim 10, used for a display portion.
 14. A method for manufacturing alight emitting device, comprising the steps of: forming a layer whichcontains a compound including a conjugated double bond and having amolecular weight of 100 to 1000; and irradiating the layer with light soas to cause a [2+2]cycloaddition reaction of the compound.
 15. Themethod for manufacturing a light emitting device according to claim 14,wherein said compound is selected from the group consisting ofanthracene, an anthracene derivative, cinnamic acid, a cinnamic acidderivative, a coumarin derivative and a pentacene derivative.
 16. Themethod for manufacturing a light emitting device according to claim 14,wherein a method for forming said layer is selected from the groupconsisting of an evaporation method, a coating method, and a drawingmethod.
 17. A method for manufacturing a light emitting device,comprising the steps of: forming a first layer which contains a compoundincluding a conjugated double bond and having a molecular weight of 100to 1000; forming a second layer by irradiating the first layer withlight so as to cause a [2+2]cycloaddition reaction of the compound; andforming a third layer over the second layer.
 18. The method formanufacturing a light emitting device according to claim 17, whereinsaid compound is selected from the group consisting of anthracene, ananthracene derivative, cinnamic acid, a cinnamic acid derivative, acoumarin derivative and a pentacene derivative.
 19. The method formanufacturing a light emitting device according to claim 17, wherein amethod for forming said first layer is selected from the groupconsisting of an evaporation method, a coating method, and a drawingmethod.
 20. The method for manufacturing a light emitting deviceaccording to claim 17, wherein a method for forming said third layer isselected from the group consisting of an evaporation method, a coatingmethod, and a drawing method.